The present invention relates to a radar device including means for generating a carrier signal with a carrier frequency fT, means for generating pulses with a pulse repetition frequency fPW, means for distributing the carrier signal to a transmission branch and a receiving branch, means for delaying the pulses, means for modulating the carrier signal in the transmission branch using the delayed pulses and for generating a reference signal, means for mixing the reference signal in the receiving branch with a received signal, and means for integrating the mixed signal. The present invention also relates to a method of suppressing the interference in a radar device including the steps: generating a carrier signal having a carrier frequency fT, generating pulses having a pulse repetition frequency fPW, distributing the carrier signal to a transmission branch and a receiving branch, generating the pulses, modulating the carrier signal in the transmission branch using the undelayed pulses, modulating the carrier signal in the receiving branch using the delayed pulses and generating a reference signal, mixing the reference signal in the receiving branch with a received signal, and integrating the mixed received signal.
Radar devices and methods according to the related art are used, for example, in short-range sensing systems in motor vehicles. They are used, for example, to prevent accidents or to detect objects in a blind spot of a motor vehicle.
FIG. 1 shows a schematic view of the basic structure of a radar device of the related art. A local oscillator (LO) 110 generates a carrier frequency fT. This carrier frequency is distributed by a power divider 116 to a transmission branch and a receiving branch. In addition to carrier frequency fT, a pulse generator 112 provides a pulse repetition frequency fPW to modulate the carrier frequency. In the transmission branch, this modulation occurs using switch 120, to which the carrier frequency is applied and which is switched with the pulse repetition frequency. The signal thus generated is emitted by a transmitting antenna 136. A modulation also occurs in the receiving branch. However, the pulses of the pulse repetition frequency are delayed by a delaying device 118 for the purpose of this modulation. These delayed pulses are used to modulate carrier frequency fT by operating switch 122, to which the carrier frequency is also applied. In this way, a reference signal SR is made available in the receiving branch. This reference signal is mixed in a mixer 124 with a received signal received via receiving antenna 134. The output signal of mixer 124 is supplied to an integrating means 126, for example, a low-pass filter and an amplifier. The signal thus generated is supplied to a signal analyzer and controller 138, preferably after analog/digital conversion. Signal analyzer and controller 138 now determines the delay of delaying device 118, which is varied between a value xcex94tmin and xcex94tmax. For example, the delay may be varied by a microcontroller or by a digital signal processor. It is also conceivable that special hardware is used for this purpose. If the transit time of the radar pulses, which as a rule is equal to twice the transit time between the target and antenna, is identical to the delay, the amplitude of the output signal of mixer 126 is at its maximum. A correlation receiver is thus available via which the distance to the target and the radial speed between the target and antenna may be inferred from the delay set by controller 138. By way of example, FIG. 1 shows only the formation of the in-phase (I) signal. The quadrature (Q) signal is formed in an analogous manner by mixing with the carrier frequency, which is 90xc2x0 out of phase.
It is basically desirable to suppress interference signals originating from highly varied sources. The use of additional modulation of the microwave signal to separate the signal components reflected by the targets from interference signals has already been described. Such methods in particular suppress interference by other uncoded transmitters, broadcast transmitters for example, or noise.
However, radar devices are also subject to noise resulting from parasitic effects which are essentially independent of the effect of other radar sensors. Thus, for example, switches 120, 122 in FIG. 1 have in reality a finite ratio between the resistances in the off or on condition Roff/Ron. In addition, undesirable emissions or bridging of the carrier frequency arise from the local oscillator, for example, to the reference input of the mixer. This means that an approximately continuous leakage signal having the carrier frequency and low amplitude is transmitted in the transmission pauses between the radar pulses. This leakage signal in particular is also present irrespective of the delay set in the reference branch and is mixed with the received signal. As a result of this and other parasitic effects, an interference signal is received in addition from targets located outside the distance range (range gate) momentarily set by the delay in the reference signal. If such xe2x80x9cundesirablexe2x80x9d targets have a large backscattering cross-section or they are within short range of the sensor, then the interference signal amplitude may be on the order of magnitude of the desired signal amplitude or exceed it and consequently result in measurement errors.
It is possible to improve the Roff/Ron ratio and accordingly reduce the interference signal amplitude by using, for example, several switches linked in series. However, this increases the technical complexity and consequently the costs.
According to a first embodiment, the present invention builds on a radar device of the related art by providing means for binary phase shift keying (BPSK) modulation of the carrier signal. BPSK modulation of the carrier signal may be used to integrate interference signals with constantly alternating signs in the subsequent integration while the desired signal is integrated with a constant sign. The interference signals are suppressed in this manner.
According to a second embodiment, the present invention builds on a radar device of the related art by providing means to switch the polarity of the received signal. In this manner, the subsequent integration suppresses the interference signals to a great extent while the desired signals are further processed.
Preferably, means are provided for BPSK modulation of the carrier signal in the transmission branch. In this variant, the carrier signal in the receiving branch may be supplied to the mixer as a reference signal without BPSK modulation. However, modulation takes place in the receiving branch so that the information necessary for the interference signal suppression is present there.
However, it may also be advantageous to provide means for BPSK modulation of the carrier signal in the receiving branch. In this case, a BPSK-modulated carrier signal is used as a reference signal while the transmitted signal is transmitted unmodulated. The information necessary for the interference signal suppression is contained in the carrier signal in the receiving branch.
It is useful in particular if the BPSK modulation results in a switchover of the phase angle for half a period TPW of pulse repetition frequency fPW. In this way, the phase of the modulated carrier signal is switched between 0xc2x0 and 180xc2x0 after each half period. This periodic switchover of the phase angle advantageously ensures that the interference signals are integrated with a constantly alternating sign while the desired signal is integrated with a constant sign. Referring to two periods in each case, a pulse is generated in the transmission branch in each of the first and second half periods TPW and in the receiving branch in each of the first and fourth half periods. The process is repeated after every two periods.
For effective interference signal suppression, it is advantageous in particular if the mixed signal is integrated over 2n periods TPW of pulse repetition frequency fPW, n being an integer equal to 1, 2, 3, . . . . This ensures that the interference signals are integrated alternately and accordingly suppressed.
It is useful if the ratio between carrier frequency fT and pulse repetition frequency fPW is an integer. This may be attained by dividing the carrier frequency by an integer. Another possibility for having the ratio as an integer is to generate the carrier frequency by multiplying an oscillator frequency with an integer and to generate the pulse repetition frequency by dividing the same oscillator frequency by an integer. The ratio between the carrier frequency and the pulse repetition frequency being an integer provides an effective interference signal suppression since the start and end of the pulse always coincide with a defined phase angle of the carrier signal.
It may also be advantageous to provide means for the BPSK modulation of the carrier signal in the transmission branch and in the receiving branch, to switch the phase angle in the receiving branch as a result of the BPSK modulation for a period TPW of pulse repetition frequency fPW and to switch the phase angle in the transmission branch as a result of the BPSK modulation in every second pulse period of pulse repetition frequency fPW and in the transmission and receiving branch for the length xcfx84 of each pulse. This makes it possible to suppress even external interference signals in addition to the interference signals based on parasitic effects.
Furthermore, it may be useful if switching means are provided to switch the polarity of the received signal. Such hardware-based polarity switching is suitable for ensuring the integration of the interference signal with an alternating sign.
However, it may also be useful if the polarity of the received signal is switched digitally. Such digital and preferably program-controlled polarity switching after analog/digital conversion reduces the hardware complexity. The integration in this case is expediently digital, for example by decimation, i.e., low-pass filtering and subsequent reduction of the sampling rate. In this case, an external low-pass is used to suppress aliasing. However, with this digital method, the I signal or the Q signal must be sampled at a high bandwidth B (B greater than fPW) and a correspondingly high sampling frequency and further processed digitally. This in turn requires additional hardware complexity.
According to a first embodiment, the present invention builds on the method of the related art in that binary phase shift keying (BPSK) modulation of the carrier signal occurs. BPSK modulation of the carrier signal may be used to integrate interference signals with a constantly alternating sign in the subsequent integration while the desired signal is integrated with a constant sign. The interference signals are suppressed in this manner.
According to a second embodiment, the present invention builds on the method of the related art in that the polarity of the received signal is reversed. In this manner, the subsequent integration suppresses the interference signals to a great extent while the desired signals are further processed.
Preferably, a BPSK modulation of the carrier signal occurs in the transmission branch. In this variant, the carrier signal in the receiving branch may be supplied to the mixer as a reference signal without BPSK modulation. However, a modulation takes place in the transmission branch so that the information necessary for the interference signal suppression is present there.
However, it may also be advantageous that a BPSK modulation of the carrier signal occurs in the receiving branch. In this case, a BPSK-modulated carrier signal is used as a reference signal while the transmitted signal is transmitted unmodulated. The information necessary for interference signal suppression is contained in the carrier signal in the receiving branch.
It may also be useful if the BPSK modulation results in a switchover of the phase angle for half a period TPW of pulse repetition frequency fPW. In this way, the phase of the modulated carrier signal is switched between 0xc2x0 and 180xc2x0 after each half period. This periodic switchover of the phase angle advantageously ensures that the interference signals are integrated with a constantly alternating sign while the desired signal is integrated with a constant sign.
Preferably, the mixed signal is integrated over 2n periods TPW, n being an integer equal to 1, 2, 3, . . . of pulse repetition frequency fPW. This ensures that the interference signals are integrated alternately and thus suppressed.
It is useful that the ratio between carrier frequency fT and pulse repetition frequency fPW is an integer. This may be attained by dividing the carrier frequency by an integer. Another possibility for having an integer ratio is to generate the carrier frequency by multiplying an oscillator frequency with an integer and to generate the pulse repetition frequency by dividing the same oscillator frequency by an integer. The ratio between the carrier frequency and the pulse repetition frequency being an integer provides an effective interference signal suppression since the start and end of the pulse always coincide with a defined phase angle of the carrier signal.
Also, it may be advantageous if a BPSK modulation of the carrier signal occurs in the transmission branch and in the receiving branch, if the phase angle is switched in the receiving branch as a result of the BPSK modulation for a period of pulse repetition frequency fPW and if the phase angle is switched in the transmission branch as a result of the BPSK modulation in every second pulse period of pulse repetition frequency fPW and in the transmission and receiving branch for the length xcfx84 of each pulse. This makes it possible to suppress even external interference signals in addition to the interference signals based on parasitic effects. In this embodiment, one pulse is generated in the transmission branch and one in the receiving branch in each period TPW of pulse repetition frequency fPW.
Preferably the polarity of the received signal is switched by switching means. Such hardware-based polarity switching is suitable to ensure the integration of the interference signal with an alternating sign.
It may also be advantageous if the polarity of the received signal is switched digitally. Such digital and preferably program-controlled polarity switching after analog/digital conversion reduces the hardware complexity. Integration in this case is expediently digital, for example by decimation, i.e., low-pass filtering and subsequent reduction of the sampling rate. In this case, an external low-pass is used to suppress aliasing. However, with this digital method, the I signal or the Q signal must be sampled at a high bandwidth B (B greater than fPW) and a correspondingly high sampling frequency and further processed digitally. This in turn requires additional hardware complexity.
The present invention is based on the surprising knowledge that it is possible to suppress interference by parasitic effects in radar devices actually constructed with relatively little technical complexity. The use of a BPSK modulation or switching the polarity of the received signal makes it possible to integrate the interference signals with constantly alternating signs while the desired signal is integrated with a constant sign. As an advantageous embodiment in particular, it should be mentioned that it is not only possible to suppress interference caused by parasitic effects in both the transmission branch and the receiving branch, but rather it is also possible to suppress external interference signals.